There are a multitude of filtration devices which separate a feed stock into filtrate and retained suspended matter which is too large to pass through the pore structure of the filter. A straight-through filter retains the suspended matter on the filter surface or within the filter matrix and passes only the filtrate. Cross-flow filters operate with tangential flow across the filter surface to sweep away suspended matter unable to pass through the filter surface pores. Cross-flow filters provide for the continuous extraction of retentate, or concentrated suspended matter, from one portion of the device and continuous extraction of filtrate from another portion. As is well known in the art, the filtration rate of cross-flow filters is generally limited by the resistance of a filter cake that builds up on the filter surface. The thickness and corresponding resistance of this cake is controlled by the cross-flow velocity. This phenomenon of cake thickness controlled by concentration polarization of retained suspended matter is extensively described in the technical literature. In order to obtain the maximum filtration rate, cross-flow filters are normally constructed from porous materials which have a low resistance to filtrate flow relative to that of the filter cake. That is, in operation the pressure drop across the porous filter itself is low relative to the pressure drop across the filter cake, and the resistance of the latter is determined by hydrodynamic flow conditions across the filter surface.
Cross-flow filters can be constructed using multiple-passageway, porous monoliths. Such monoliths can have tens to thousands of passageways extending through them, with the passageways normally parallel and uniformly spaced. When in use the feed stock is introduced under pressure at one end of the monolith, flows in parallel through the passageways, and is withdrawn as retentate at the downstream end of the device. Filtrate which passes into the porous monolith walls separating the passageways combines and flows through the walls toward the periphery of the monolith, and is removed through an integral, pressure-containing outer skin of the monolith. The resistance to flow in the tortuous flow path of the monolith passageway walls can severely limit filtration capacity, and for this reason cross-flow filters based on high surface area, multiple-passageway, porous monoliths are not found in commercial use.
Membrane devices utilize a semipermeable membrane to separate filtrate, also called permeate, from retentate. There is a multitude of different membrane devices which separate and concentrate particles, colloids, macromolecules, and low molecular weight molecules. Membranes generally require a mechanical support which can be integral with the membrane, as for self-supporting asymmetric membranes, or separate. For the latter, membranes can be coated onto, or simply mechanically supported by, a porous support material.
Multiple-passageway, porous monoliths can be especially useful as membrane supports. In this instance membranes are applied to the passageway walls, which serve both as a mechanical support and as the flow path for filtrate removal to a filtrate collection zone. A high flow resistance of the passageway walls of the monolith can be troublesome first in that it can prevent adequate formation of membranes, for example, by dynamic formation procedures. Second, if membranes are otherwise applied to the monolith passageway walls, the resistance of the passageway walls to filtrate flow can limit device capacity. This limitation has clearly been recognized by developers of such devices, for example, by Hoover and Roberts in U.S. Pat. No. 4,069,157. That patent teaches a solution to such limitation by limiting a number of parameters to values within specific ranges. The surface area of the passageways per unit volume, the porosity of the support, and the proportion of the volume of the support material exclusive of the passageways to the total volume of the support are defined within certain ranges, and are combined to define an allowable range of a permeability factor for the support.
Other monolith-based membrane devices have been developed in the United States, France, and The People's Republic of China. For these devices practitioners also have recognized a support permeability limitation and have generally overcome this limitation by use of monoliths with combinations of small overall diameter, relatively few feed passageways and large pore size of the support material. One commercially-available membrane device utilizes a number of small diameter hexagonal monoliths, each with up to 19 passageways, distributed within a cylindrical housing. Filtrate exits from all six sides of each monolith and mixes with the filtrate from the other monoliths, after which it is collected. The overall packing density, or membrane area per unit volume, of this device is quite low.
The monoliths used by all the above sources as supports for membrane devices have had the common characteristic of employing passageways which are substantially uniformly spaced throughout the support. Given this constraint, product developers have worked with variables such as those detailed by Hoover and Roberts in the above referenced patent to avoid filtrate flow path limitations.
Thus the flow resistance of the passageway walls of porous monoliths can be a limiting factor in the use of monoliths either as cross-flow filtration devices or as membrane supports in membrane devices. Further, this limitation becomes increasingly severe as the packing density, or effective filter or membrane area per unit volume, of the device increases.
In the field of heat exchangers there are several conventional devices having a multiple flow path body. For the device of Kelm, U.S. Pat. No. 4,041,592, for example, two fluids enter separately into a body, are maintained separately within the body, and exit separately. Thermal exchange occurs between the two fluids but there is no transfer of matter. Kelm suggests that a porous body can be utilized for the exchange of matter or the filtering of a fluid between flow paths, but no further teaching is provided.